This invention relates to attenuators and more particularly to attenuators employing phase modulation.
There are many applications which require attenuators of a signal. One class of applications are transmitters employing high gain stages of amplification such as traveling wave tubes operating in saturation or nonlinear amplification components and requiring that a continuous wave (CW) signal be transmitted at low and high power levels. To transmit at low power the CW signal must first be attenuated prior to being applied to the amplification stage. Because of the saturated operation of the amplification stage or the nonlinearity thereof it cannot be determined for certain how much attenuation of the signal prior to amplification will produce the desired attenuation of the output from the amplification stage. Accordingly, other means must be used to insure correlation between the amount of attenuation of the input signal and that of the amplified signal. Typically, automatic gain control (AGC) is employed. This, however, is undesirable as the AGC circuit increases complexity, size, expense, power requirements, etc., of the overall system. Also, the level detector of the AGC circuit contributes to inaccuracy by its variation with frequency and temperature.
Furthermore, typical attenuators employed in such a system occasion other problems. If diode attenuators are used then all the problems generally associated with diodes come into being, such as their switching speed and their dependence on temperature, frequency and the like. An alternate to the diode attenuators is the electron beam switching modulator (EBSM) tube, a TWT whose gain can be varied by a control grid voltage. This solves many of the problems associated with diodes, however, these tubes are expensive, bulky and unreliable in that they have the high failure rate associated with tubes in general and with high voltage supplies required to operate such tubes.
One method of avoiding the problem of nonlinear attenuation for the amplification stage without using AGC is to always transmit full power or no power, during the attenuated portion of the signal, by switching the signal off and on at a rate much higher than the bandwidth of any receiver to receive the signal. However, this substitution of duty factor for attenuation has many problems of its own in that it requires extremely fast diode switches. In one example if the receiver has an i.f. bandwidth of 10 megahertz then a switching rate of at least 40 megahertz would be required to make the effect of the switching undistinguishable from a true attenuation by the receiver. And if a 20 dB attenuation is required, then a pulse width of 2.5ns must be produced. This means that the rise and fall times of the switching pulses must be on the order of less than one nanosecond.